Fix Unhandled ReferenceError in debug-debugger.js

This fixes following exception in Sky on attempt to set a breakpoint
"Unhandled: Uncaught ReferenceError: break_point is not defined"
I think this happens in Sky but not in Chrome because Sky scripts are executed in strict mode.

BUG=None
LOG=N
R=yangguo@chromium.org

Review URL: https://codereview.chromium.org/741683002

Cr-Commit-Position: refs/heads/master@{#25415}

src/debug-debugger.js

@@ -432,7 +432,7 @@ ScriptBreakPoint.prototype.set = function (script) {
   if (IS_NULL(position)) return;
 
   // Create a break point object and set the break point.
-  break_point = MakeBreakPoint(position, this);
+  var break_point = MakeBreakPoint(position, this);
   break_point.setIgnoreCount(this.ignoreCount());
   var actual_position = %SetScriptBreakPoint(script, position,
                                              this.position_alignment_,